* Magicloud Magiclouds <magicloud.magiclouds@gmail.com> [2014-07-28 13:46:53+0800]

> Hi,
>
>   For code like below, how to make it compilable?
>
> data EventHandlers = forall m. MonadIO m => EventHandlers { onDeleteWindow
> :: Maybe (m ()) }
> instance Default EventHandlers where
>   def = EventHandlers Nothing

GHC has to store some MonadIO dictionary in the existential type, but it cannot
figure out which dictionary to store (it can be any).

You can tell it which one to use by supplying a type annotation, e.g.:

  def = EventHandlers (Nothing :: Maybe (IO ()))

Roman